Field emission devices are microscopic electrical components which selectively emit electrons. Such devices 100, as shown in FIGS. 1a and 1b, generally comprise two electrodes: an emitter electrode 103 for emitting electrons and a gate electrode 104 for controlling the flow of electrons from the emitter electrode 103 depending on the electrical charge present at the gate 104. The electrodes are typically mounted on some kind of substrate 101 or 105 to provide support for the device, with a gap between the electrodes. A third electrode, the anode (not shown in FIGS. 1a and 1b), may also be present to receive the emitted electrons, although in some devices the gate electrode 104 serves as the anode.
Field emission devices have been known for several years to have many potential applications in commercial and military industry, such as: high-definition television; flat-panel video displays; radiation-hard thermally insensitive integrated circuits; microsensors; fast electron sources for vacuum tubes; and electron microscopes. However, there are a number of practical difficulties associated with such devices which have inhibited their widespread use. Three such problems are 1) their extreme sensitivity to damage, 2) their instability evidenced by a tendency towards microstructure changes with use, and 3) the difficulty of manufacturing such devices with sufficient uniformity and reproducibility. The following references detail these problems, and describe the state of the prior art in the manufacture of emission devices.
U.S. Pat. No. 3,947,716 discloses a field emission tip and process wherein a metal adsorbate is selectively deposited on the tip to create a selectively faceted tip with the emitting planar surface having a reduced work function and the non-emitting planar surfaces having an increased work function, thus yielding improved performance. The patent discloses the use of a single crystal to fabricate emission tips, but the reason for single crystal use in emission tips has traditionally been to facilitate fabrication of a cone-shaped emitter. The patent does not mention the use of single crystals for the other electrodes of the device, nor does it suggest the use of single crystals in conjunction with thin film emitters or for stability and arc damage resistance.
S.M. Spitzer and S. Schwartz, "A Brief Review of the State of the Art and Some Recent Results on Electromigration in Integrated Circuit Aluminum Metallization", I. Electrochem. Soc. v. 116, p. 1368 (1969), discusses some of the problems associated with electromigration in integrated circuit devices. Electromigration phenomena have been found to cause instability and susceptibility to damage in emission devices. The article does not mention the use of single crystal material to reduce electromigration problems.
J. E. Wolfe, "Operational Experience with Zirconiated T-F Emitters", I. Vac., Sci. Technology. v. 16, p. 1704 (1979), discusses the characteristics of an electron gun which uses a cathode-filament structure with a needle-shaped cathode. It discusses some techniques for improving performance and extending device lifetime, but does not mention grain boundaries or single-crystal structures.
G. W. Jones, C. T. Sune, and H. F. Gray, "Self-Aligned Vertical Field Emitter Devices Fabricated Utilizing Liftoff Processing", 3d Int'l Vacuum Microelectronics Conf., Jul. 23-25, 1990, Monterey, Calif., poster 1-2, sets forth a method of fabricating vertically self aligned field emitter cathodes and extraction electrodes utilizing liftoff process and anisotropic silicon etching. This technique involves first forming silicon dioxide islands on heavily doped N+ silicon and then using those islands as etch masks to form flat topped pyraraids with silicon dioxide overhanging caps.
R. B. Marcus et al., "Formation of Sharp Silicon and Tungsten Tips", 3d Int'l Vacuum Microelectronics Conf., Jul. 23-25, 1990, Monterey, Calif., paper 1-3, describes a variation on a previously known procedure for forming atomically-sharp silicon tips of between 10.degree. and 15.degree. half-angle by utilizing oxidation inhibition at regions of high curvature for silicon tips. The variation employs a chemical vapor process to form similar tips out of tungsten.
K. Warner, N. M McGruer, and C. Chan, "Oxidation Sharpened Gated Field Emitter Array Process", 3d Int'l Vacuum Microelectronics Conf., Jul. 23-25, 1990, Monterey, Calif., poster P-25, discusses a process for fabricating gated field-emission cathodes with sharp tips by oxidation.
D. W. Branston and D. Stephani, "Field Emission from Metal Coated Silicon Tips", 3d Int'l Vacuum Microelectronics Conf., Jul. 23-25, 1990, Monterey, Calif., paper 5-4, describes emission properties of various groupings of emitters formed as arrays of silicon tips coated with various refractory metals by physical vapor deposition techniques.
The methods set forth in the above-referenced articles generally represent the state of the art in manufacturing techniques for emission devices.
S. Bandy, C. Nishimoto, R. LaRue, W. Anderson, and G. Zdasiuk, "Thin Film Emitter Development", Technical Digest of IVMC 91 (August, 1991), p. 118, published within one year of the instant patent application, describes an emission device manufacturing method using thin films. It sets forth the properties and advantages of thin film emitters in comparison with traditional cone-shaped emitters. These two structures for emission devices are shown in FIGS. 1a and 1b of the instant patent application. FIG. 1a shows a well-known cone emitter structure, in which a cone-shaped emitter electrode 103 is mounted on a conducting substrate 101 (as stated in "Thin Film Emitter Development", "virtually all structures reported in the literature use conducting substrates."). Devices of this type are commonly manufactured using etching or metal closure techniques. FIG.1b shows the newer "edge emitter" structure discussed in "Thin Film Emitter Development", in which an edge of the emitter 103 protrudes from between an insulator 102 and a metal overlay 106. This structure usually employs an insulating substrate 105. Edge emitters offer several potential advantages over cone-shaped emitters, including improved reproducibility and uniformity, high current densities, and high frequency performance. Even with these advantages, however, the three problems mentioned above persist.
Although it has been known in the art for some time that the use of single crystals facilitates fabrication of cone-shaped emitter electrodes, the benefits of single crystals in improving stability and uniformity and reducing damage have not been previously known. Accordingly, they have not been used for the other electrodes of the device (namely the gate and the anode), nor have they been used for non-cone-shaped emitters. None of the prior art suggests the novel features of the present invention, in which single crystals are used to form some or all of the electrodes of the device, not just cone-shaped emitters, in order to alleviate the problems of uniformity, reproducibility, stability, and sensitivity to damage.